As a result of improvement in processing technology, it is now possible to put millions of transistors in an integrated circuit. This increases the amount of processing power of the integrated circuit. However, the processing power may be wasted if there are insufficient input-output (I/O) resources to transfer data to and from the integrated circuit. High speed I/O transceivers alleviate this problem by increasing the data transfer speed of the I/O resources.
A transceiver contains a serializer and a deserializer. A multi-gigabit serializer takes vectors of bits (e.g., 20 bits wide) and converts them to a serial (one bit at a time) bit stream for high-speed transmission over a single differential pair connection. Conversely, a deserializer receives a serial bit stream on a separate differential pair and converts it into vectors of bits. The two circuits together are called a SERDES (serializer/deserializer).
The rate at which bits are transmitted or received on the differential pair connection is called the serial baud rate, commonly measured in Gbps (gigabits per second), or perhaps Mbps (megabits per second). A number of communication standards exist involving the use of SERDES transceivers. The serial baud rates for some common standards of interest range from 1.06 Gbps to 3.125 Gbps. Other potential applications may use lower serial baud rates, e.g., 622 Mbps or lower. It is desirable for a single transceiver to be able to operate over this entire range of serial baud rates, especially when it is embedded in a programmable logic device such as a field-programmable gate array (FPGA), which is intended to be used for a broad range of applications.
It is difficult for conventional circuits like the SERDES to operate at high speed. Thus, many new circuits are specially designed to handle the challenge. Further, the values of components need to be fine-tuned and carefully selected because the SERDES must meet exacting specifications over its entire range of serial baud rates. One result is that a transceiver can optimally operate in a narrow speed range. As an example, some SERDES are designed to operate in the range 800 Mbps to 3.125 Gbps. Such a SERDES is not usable as-is for designs with serial baud rates below 800 Mbps, for example, the 622 Mbps applications noted above. Consequently, it is desirable to be able to extend the range of operation of a SERDES.